From 0416d6f05a96e0f1b3751aa97abfffe6d3323976 Mon Sep 17 00:00:00 2001
From: "Miss Islington (bot)"
 <31488909+miss-islington@users.noreply.github.com>
Date: Wed, 22 May 2019 18:41:43 -0700
Subject: [PATCH] bpo-27737: Allow whitespace only headers encoding (GH-13478)
 (#13517)
MIME-Version: 1.0
Content-Type: text/plain; charset=utf8
Content-Transfer-Encoding: 8bit

(cherry picked from commit ef5bb25e2d6147cd44be9c9b166525fb30485be0)

Co-authored-by: Batuhan Taşkaya <47358913+isidentical@users.noreply.github.com>
---
 Lib/email/header.py                                            | 2 +-
 Lib/test/test_email/test_email.py                              | 3 +++
 .../next/Library/2019-05-22-02-25-31.bpo-27737.7bgKpa.rst      | 2 ++
 3 files changed, 6 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2019-05-22-02-25-31.bpo-27737.7bgKpa.rst

diff --git a/Lib/email/header.py b/Lib/email/header.py
index 7b30a039da1c..4ab0032bc661 100644
--- a/Lib/email/header.py
+++ b/Lib/email/header.py
@@ -431,7 +431,7 @@ class _ValueFormatter:
         if end_of_line != (' ', ''):
             self._current_line.push(*end_of_line)
         if len(self._current_line) > 0:
-            if self._current_line.is_onlyws():
+            if self._current_line.is_onlyws() and self._lines:
                 self._lines[-1] += str(self._current_line)
             else:
                 self._lines.append(str(self._current_line))
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 621754cf753d..dfb3be84384a 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -4964,6 +4964,9 @@ A very long line that must get split to something other than at the
         msg['SomeHeader'] = '   value with leading ws'
         self.assertEqual(str(msg), "SomeHeader:    value with leading ws\n\n")
 
+    def test_whitespace_header(self):
+        self.assertEqual(Header(' ').encode(), ' ')
+
 
 
 # Test RFC 2231 header parameters (en/de)coding
diff --git a/Misc/NEWS.d/next/Library/2019-05-22-02-25-31.bpo-27737.7bgKpa.rst b/Misc/NEWS.d/next/Library/2019-05-22-02-25-31.bpo-27737.7bgKpa.rst
new file mode 100644
index 000000000000..02d0ef891bec
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-05-22-02-25-31.bpo-27737.7bgKpa.rst
@@ -0,0 +1,2 @@
+Allow whitespace only header encoding in ``email.header`` - by Batuhan
+Taskaya
-- 
2.47.3